This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Protein degradation by the proteasome must be tightly regulated, as it controls events ranging from cell cycle progression to cell death. The 26S proteasome is composed of a 20S catalytic core particle (CP) that is capped at either end by a 19S regulatory particle (RP). Whereas the CP contains the enzymatic activity responsible for proteolyzing protein substrates, the commitment to protein degradation is determined by RP components, which recognize and process substrates prior to their passage into the CP. For most proteasome substrates, ubiquitination is prerequisite to their degradation, and their initial interaction with the proteasome is through RP ubiquitin receptors, S5a and Rpn13. Substrates are subsequently deubiquitinated by three deubiquitinating enzymes and unfolded by a heterohexomeric ring of ATPases. These activities prepare substrates for entry through a narrow chamber leading to the catalytic center of the proteasome's CP. We study how substrates are processed by the 19S regulatory particle of the proteasome with a focus on substrate recognition and deubiquitination. By using NMR spectroscopy, we are studying how Rpn13 is docked into the proteasome and whether this localization activates its functional role as a ubiquitin receptor. We have devised a novel strategy to study Rpn13 and S5a in the context of the RP and test a working model of their coordinated binding to ubiquitinated substrates. We also study how each of these proteins bind to ubiquitin chains.